I. Field of the Invention
This invention relates generally to monitoring devices for respiratory activity, and more particularly to a transducer which can be readily positioned relative to the respiratory passages of a subject for producing an electrical signal related to temperature changes due to impingement of inspired or expired air thereon and/or sound produced during breathing.
II. Discussion of the Prior Art
A variety of devices are described in the prior art for monitoring respiratory activity of a subject. For example, neonates who may have evidenced a propensity towards SIDS (sudden infant death syndrome) are provided with a breathing monitor which will provide an alarm in the event of sleep apnea. In this regard, reference is made to the Pearce U.S. Pat. No. 4,971,065. There is described a transducer for detecting apnea which comprises a deformable pad having a serpentine pattern of an electrically resistive material adhered to that pad, the material exhibiting a resistance change when deformed. The transducer would typically be placed on the crib mattress to detect even subdued breathing movements of that infant. The transducer is coupled to electrical circuitry for producing an alarm if the rhythmic breathing pattern ceases for more than a predetermined time.
The Pennock U.S. Pat. No. 4,960,118 describes a transducer arrangement in which piezoelectric devices are mounted on belts extending around the abdomen and thorax in such a way that the strain on the transducers due to expansion and contraction of the abdomen and thorax during breathing produces a detectable variation in an output signal.
The EdenTec Corporation of Eden Prairie, Minn., manufactures and sells an air flow sensor designed to be adhesively attached to a patient's upper lip so as to be exposed to the tidal flow of respiratory gases during inspiration and expiration. It is more particularly described in the Bowman et al. U.S. Pat. No. 5,161,541. The transducer comprises a substrate on which is deposited a thermistor, i.e., a material exhibiting a significant resistance change with changes in temperature. The device is adapted to be connected to a suitable electronics module for developing a detectable change in voltage as the resistance varies due to temperature changes occasioned by the impingement of respiratory gases onto the transducer during inspiration and expiration. Typically, resistive printed inks have a small temperature coefficient producing a very small change in resistance of about 0.05% with the typical 1.degree. to 2.degree. C. temperature change that occurs when breathing onto the sensor. This produces a very small signal voltage output of less than 100 microvolts with a typical adult patient. The signal is even less when the sensor is used with an infant because of the smaller volume of air exhaled.
The present invention overcomes this drawback by using a piezo material that has pyroelectric properties producing output voltages that are approximately 1,000 times greater than the resistive printed ink sensor. These larger signals make it easier to detect small changes of temperature.